Is there a way to control how exponentials are factored

I have a long messy expression that I'm trying to simplify.   On thing
that would help would be to better control how sub expressions
containing exponentials are factored.   Consider the following:

(%i1) factor(exp(a)+exp(b));
                                     b     a
(%o1)                              %e  + %e

(%i2) factor(exp(a)+exp(-b));
                                - b    b + a
(%o2)                         %e    (%e      + 1)


Both are mathematically correct, but how maxima chooses b is often very
unhelpful, and in any event is completely arbitrary given that either a
or b could have either sign.   Is there a flag that controls this
"simplification"?

David